A problem of high temperature fuel cell systems for small power classes is that as a result of the low in-system heat production, it is difficult to realise the high work temperatures (several 100° C.) needed for operation. Potential solutions can be considered with the installation of burners. However, efficiency reasons must be considered here. At the same time a space-saving structure must be implemented for mobile and portable systems.
Fuel cells are tertiary galvanic elements that have been known for some time. Among the various fuel cell types, the solid oxide fuel cells occupy an outstanding position due to the fact that they have the highest fuel flexibility. Because of the high work temperature that generally exceeds 600° C., thermal losses are however of critical importance, especially with small systems. This is also the reason why the majority of the SOFC applications is not designed for small mobile or portable systems (Fuel Cell Handbook 7th edition, EG&G Services, Inc. U.S. Department of Energy, Office of Fossil Energy, National Energy Technology Laboratory, Morgantown, West Va., November 2004; Handbook of Fuel Cells Fundamentals, Technology and Application, Wolf Vielstich, Hubert A. Gasteiger, Arnold Lamm, 2003 John Wiley & Sons, Ltd.).
Among the solid oxide fuel cells (SOFCs) the microtubular designs are particularly stable against variations in temperature (V. Lawlor, S. Griesser, G. Buchinger, A. Olabi, S. Cordiner, D. Meissner—Review of the micro-tubular solid oxide fuel cell (part I: Stack design issues and research activities)—Journal of Power Sources, 2009, pp. 387-399.).
The task of the invention is to secure improved utilisation and stable operation of fuel cells—especially microtubular solid oxide fuel cells—also in the case of variations in temperature and temperature gradients occurring during operation, and also to provide a system that is as compact as possible, especially for mobile and portable applications.
This task is solved in accordance with the invention by a fuel cell system that has the features claimed herein.
The individual characteristics of the design examples, which are described in the following, can be realized independently from each other within the framework of the present invention, thus do not have to be exactly implemented in the characteristic combinations presented in the examples.
The invention enables direct warming of in particular microtubular fuel cells because of convection, radiation and in particular thermal conduction.
The special stability of the microtubular SOFCs against temperature gradients can be used to solve this problem.
In a fuel cell system in accordance with the invention at least one burner is directly arranged at a fuel cell and/or one burner is a separate element of a fuel cell system with several fuel cells.
There can be at least one microtubular SOFC in a system in accordance with the invention whereby one or more burners are located in the direct vicinity of a fuel cell.
One or more burners can be executed directly as an integral part of one or more fuel cells.
One or more burners can be present directly as an integral element of one or more fuel cells, whereby this/these can be integrated as part of a sealing concept for separating the gas chambers between an atmosphere with oxidizable gas and an atmosphere with reducible gas. The gas chambers are separated in such a way that a partial gas exchange between oxidizable and reducible gas can take place that leads to a desired exothermic chemical reaction and thus to warming or heating up.
To form the burner at least one aperture in the layer of a gas-tight electrolyte can exist or be formed/attached there. An electrolyte can also be completely or partially open porous so that in both cases gas exchange and consequently an exothermic reaction can be achieved.
This/these aperture(s) can be present already in the manufacturing process of the electrolyte layer or be formed during this. At least one aperture can also be inserted after the electrolyte layer has already been produced. This also applies for porous areas. Apertures of the electrolyte can be formed, for example by application (brushing, printing, spraying of a layer of an organic material such as e.g. waxes at the positions that are subsequently to display a aperture or porosity. After application of these organic layers the actual material forming the electrolyte can be applied (e.g. dip coating, electrophoretic deposition, spraying, splattering, plasma coating, slip casting . . . ). After this a sintering can be carried out, whereby at least one aperture or increased porosity is formed as a result of the burnout of the organic layer. It is possible to proceed by analogy when forming the outer electrode, whereby direct electronically conductive contact between the inner and outer electrodes should be avoided. Alternatively, specifically the electrolyte and the outer electrode can be applied at the desired positions by printing, spraying or other ceramic processes. The porosity can for example be set by adding a pore former, such as e.g. carbon or PMMA, whereby it is also possible to produce a graduated transition of the pore ratio up to a tight electrolyte. The pore ratio or the porosity changes successively there. In a special embodiment sealing materials (e.g. glass solders, ceramic adhesives, metal solders . . . ) can be applied directly next to the positions at which the exothermic reaction through the gas exchange is to take place. These sealing materials should be placed next to the electrolyte or even overlap it in such a way that the electrolyte is not directly arranged in the area heated by the exothermic reaction. The advantage of this arrangement lies in the fact that the electrolyte and possibly also the outer electrode is not directly next to the hottest point (hot-spot), the position at which the targeted combustion of an oxidizable component takes place, so that lower thermal stresses occur at the electrochemically active centres.
Apertures or porous areas can preferably be present only at the ends or only in the middle (possibly as interruptions of an outer electrode coating) of the microtubular SOFCs. They can also be present at the ends and in the middle of the microtubular SOFCs. One or more strip-shaped apertures or porous areas can run along the cell axis or at an angle of 0 to 179° to the cell axis along a part of the cell area usually covered by electrolytes or other approximately gas-tight material. Combinations of such patterns of deliberately produced gas leakiness between at least one reducing and at least one oxidizing atmosphere along the cell or its seals can also be used in the invention.
Those areas of the fuel cells that are not provided with a gas-tight electrolyte layer or directly adjacent to this can consist of another material. Areas of different materials or porosities can for example be produced by a sequential injection moulding method or by joining using (possibly electrically conductive) ceramic adhesives, glasses or metal solders.
Any material customarily used as an ion conductor in the field of the fuel cells—such as e.g. polymer electrolytes—can be used as electrolyte. Materials for solid oxide fuel cells such as e.g. doped zirconium oxides, doped cerium oxides and doped gallates are particularly preferred. Materials or material combinations used in the field of fuel cell research are equally suitable for the electrodes. For the cathodes these are for example noble metals (Pt, Rh, Pd, silver, . . . ), ceramic perovskites (LSM, LSC, LSCF, LSF, . . . ), nickelates and composite electrodes made of cathode material and ion conductors. For the anodes it is possible to use e.g. noble metals (Pt, Rh, . . . ), transition metals (nickel, copper, . . . ) or ceramic compounds such as perovskites, titanates, chromites and niobates.
If the exothermic reaction takes place with oxidizable components at sealing elements, any sealing concepts that are used in the field of fuel cell technology can be used. Examples of this are ceramic adhesives or glasses in which pores are formed selectively by adding pore forming agents, by insufficiently long drying after application or also where appropriate subsequently by mechanical or chemical means. Alternatives as sealing elements are also pressure seals in which the desired gas leakiness can be set by varying the compressive force, it is possible to vary said compressive force depending on the system state (e.g. higher leakiness during heating up, maximum tightness during cooling down). The gas exchange of the gases/gas mixtures used for an exothermic reaction can also be influenced selectively by bringing influence to bear via gap dimensions, perforated sheets or lamellar seals within a partition wall between an oxidizing and a reducing atmosphere in which the cells are embedded.
Not all the cells or seals in a fuel cell system with more than one fuel cell have to function as burners.
Parts functioning as burners can consist of a different material combination than the parts not functioning as burners.
The invention particularly preferably uses microtubular SOFCs that have a diameter of 0.01 mm-20 mm (preferably between 0.5 mm-5 mm) and a length of 1 mm-500 mm (preferably between 10 mm-100 mm). These solid oxide fuel cells can be electrolyte based (ESC), anode based (ASC), cathode based (CSC) and metal based (MSC). ESCs, CSCs and MSCs tend to show enhanced stability to reoxidation of the anode. One of the particularly preferred embodiments is the use of MSCs, as the metal substrate conducts the heat resulting from targeting exothermic reaction well and this consequently leads to fast and as far as possible uniform warming of the cell. In the case of ASCs it is ensured through appropriate flow control that the anode does not oxidize or only oxidizes locally and the gas-tight electrolyte is not damaged. It is also possible to prevent or at least alleviate the danger of cell damage caused by deoxidizing nickel through appropriate modification of the anode structure. Such a modification consists for example in the gradual decrease of the content of oxidizable metal (e.g. nickel) from the middle of the substrate to the surface of the adjacent electrolyte or in the use of a coarse-grained porous oxide framework (preferably an ion conductor such as YSZ) in which the nickel particles were applied by impregnation. An alternative option is the use of anode materials stable to reoxidation such as it is mostly the case with ceramic anode materials, such as e.g. perovskites.
Catalytically active substances such as e.g. noble metals (Pt, Rh, Ru, . . . ) or ceramic oxides (perovskites) can be added to the anodes or to parts of the fuel cells or sealing elements functioning as burners whereby said substances can lead to an initiation of the exothermic reaction already at low temperatures and/or low concentrations of oxidizable substances (hydrogen, carbon monoxide, hydrocarbons, alcohols, ammonium, DME, . . . ) and/or trigger reforming functions too. A possible adding of catalytic substances only at the positions/areas without gas-tight electrolyte can be executed for example by spraying, impregnation, electrochemical deposition, electrophoretic deposition, printing or other customary processes.
The gas supply to the parts functioning as burners can be regulated or controlled separately. The gas supply to one electrode or both electrodes of the fuel cells can be stopped or regulated to cool the system. Thus a gas with enlarged volume flow rate can be fed in so that it is not completely consumed/converted/oxidized in a reaction and a surplus component of this gas that can be fed cold leads to cooling. A non-combustible gas (such as e.g. nitrogen or noble gases) and/or gas or gas mixture with a cooling effect through an endothermic reaction (e.g. hydrocarbons in combination with steam in any mixture) can also completely or only partially replace the originally combustible gas.
For heating up the system the gas supply to the parts functioning as burners can be actuated differently than the gas supply to the parts of a system in accordance with the invention not functioning as burners. For example an increased pressure/flow of oxidizable and/or reducible gas can be applied to the fuel cells or sealing elements for heating up the system, which leads to an increased exothermic reaction that allows faster heating up. On the other hand the flow or pressure can be reduced or even stopped at these positions/areas for cooling the system.
In the scope of this invention it is possible to arrange serial connections of one or more gas flows of cells or sealing elements functioning as burners with other fuel cells or system components (e.g. reformers) of the system and as a result intensive heat exchange is possible between the burner modules and the following components. As a result of this serial connection, fuel (hydrocarbons, alcohols, ammonia, reformate gas, DME, . . . ) reforming is also advantageously possible for the following system parts (cells, reformers, after-burners, . . . ), especially when not the whole fuel is used for burning.
A special, possibly periodic supply or periodic changes in the amount of gas with reducing effect and gas with oxidizing effect can be set for control and to avoid any damage (e.g. reoxidation) to the system.
The quantity supply of at least one gaseous and/or liquid reactant can be used to influence the temperature and/or performance of the system or parts of the system.
Of course the features set out above and those to be explained below can be used not only in the respective combination stated, but also in other combinations or as stand-alone features without leaving the scope of the present invention.
The work temperatures of typically 500° C.-1000° C. required for the operation of solid oxide fuel cells can be maintained simply and safely with the invention, which otherwise cannot be achieved easily for small systems. Heat losses can be considerably reduced or avoided. Mainly in the starting phase of a high temperature fuel cell system, the application of heat can be achieved quickly and efficiently up to the operation of the fuel cells. The application of additional burners as a matter of principle has already long been known from the state of the art (US200710243444; WO2007082522; DE19517425C1; Journal of Power Sources 86 (2000) 376-382). However, these publications in no place mention direct use of a fuel cell or a fuel cell seal as burner. The invention makes it possible to generate heat directly close to one or more fuel cells, even directly at the fuel cell or even more directly in the direct vicinity of the reactive centres of one or more fuel cells. As a result an ideal heat transfer from the exothermic reaction to the reactive areas can be achieved, which in turn reduces the fuel required for heat generation and can even lead to a decrease of the thermal insulation complexity. As a result of this property, in particular small SOFC systems can be operated efficiently. Through the combination of a fuel cell that at the same time can function as a burner there is an exceptional possibility to reduce system size, material and production costs.